ABSTRACTS 'Extreme Discharges' - CHR-KHR

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Climate Change and Hydrological Extremes Christoph Schär Atmospheric and Climate Science, ETH Zürich Winterthurerstr.190, CH-8057 Zürich, Switzerland schaer@env.ethz.ch The estimation of flood and drought frequencies is a delicate and difficult task, even in a stationary climate. Modern estimation methods utilize some probabilistic methodology and thus provide a probability density function (PDF), for instance of the frequency of a certain event category. Climate change will further complicate the task of estimation, as the assumption of a stationary climate system is rejected. The most immediate effect of climate change in this context is thus to increase the uncertainty in flood and drought estimation (corresponding to a widening of the PDF). Such an increase in uncertainty in essence implies an increase in safety margins and thus costlier protection for the same level of security. It is important to realize that this kind of effect takes place, irrespective of whether climate change will actually increase or decrease the frequency of the respective event category (corresponding to a shift of the PDF). In the long run, however, appropriate estimation methodologies should rely on a combination of past observations and climate change scenarios. Climate models have made rapid progress in recent years. Yet the projection of regional climate changes – and in particular of catchment-scale extreme events – is associated with large uncertainties. Here results of climate change scenarios will be discussed with reference to European-scale summer droughts. Anthropogenic climate change is usually perceived as a comparatively slow trend towards higher temperatures, with attendent shifts in precipitation distribution and hydrological conditions. Associated changes in interannual (year-to-year) variability are usually considered small. In a recent paper (Schär et al. 2004, see also Vidale et al. 2005), we have proposed that climate change may give rise to a pronounced increase in interannual variability during the extratropical summer season. Such an increase in variability would have important repercussions for the frequency of extreme summer heat waves and droughts, and would also represent a challenge to adaptive response strategies designed to cope with climate change. We utilize regional climate models (RCMs) from the European project PRUDENCE and provide a detailed analysis of physical processes implicating on the frequency of heat waves and droughts over Europe. RCM simulations suggest that interannual summer temperature variability may increase considerably, by up to a factor two towards the end of the current century. The main region of enhanced variability is the temperate climate zone in Central and Eastern Europe. Analysis of the simulated water cycle shows that the increase in variability is at least partly related to land-surface processes, and associated with an increasing frequency of summer droughts. The representation of the associated non-linearities requires a realistic description of the continentalscale water cycle in the coupled atmosphere / land-surface system. Intercomparison of model simulations suggests that there is considerable agreement between different models regarding the occurrence of the effect, but there are large differences regarding its geographical distribution, seasonal evolution and amplitude. These differences are related to a wide range of model characteristics, among them the representation of the soil hydrology and the simulation of large-scale circulation anomalies. A particularly important uncertainty in climate models relates to the seasonal cycle of terrestrial water storage. The study was motivated by the extreme European summer 2003. In much of Central Europe, mean summer temperatures exceeded their long-term mean by more than 4 standard deviations, leading to an extreme heat wave. A brief analysis of this event will also be presented. It will be argued that the frequency of extreme heat waves and droughts is more sensitive to an increase in temperature variability (i.e. a widening of the statistical distribution), than to a mere warming (i.e. a shift of the distribution towards warmer and dryer conditions). References: Frei, C., H.C. Davies, A. Gurtz and C. Schär, 2000. Climate Dynamics and Extreme Precipitation and Flood Events in Central Europe. Integrated Assessment, 1, 281-299 23
Frei, C., and C. Schär, 2001. Detection probability of trends in rare events: Theory and application to heavy precipitation in the Alpine region. J. Climate, 14, 1568-1584 Frei, C., R. Schöll, J. Schmidli, S. Fukutome and P.L. Vidale, 2005: Future change of precipitation extremes in Europe: An intercomparison of scenarios from regional climate models. J. Geophys. Res., submitted. Hirschi, M., S.I. Seneviratne and C. Schär, 2005: Seasonal variations in terrestrial water storage for major midlatitude river basins. J. Hydrometeorol, submitted Kleinn, J. C. Frei, J. Gurtz, D. Lüthi, P.L. Vidale, and C. Schär, 2005: Hydrologic Simulations in the Rhine Basin driven by a Regional Climate Model, J. Geophys. Res. – Atmos., 110, D04102 Schär, C., D. Lüthi, U. Beyerle and E. Heise, 1999: The Soil-Precipitation Feedback: A Process Study with a Regional Climate Model. J. Climate, 12, 722-741 Schär, C., P.L. Vidale, D. Lüthi, C. Frei, C. Häberli, M.A. Liniger and C. Appenzeller, 2004: The role of increasing temperature variability for European summer heat waves. Nature, 427, 332-336 Schär, C. and G. Jendritzky, 2004: Hot news from summer 2003. Nature, 432, 559-560 Schmidli, J., C. Schmutz, C. Frei, H. Wanner and C. Schär, 2002. Mesoscale precipitation variability in the Alpine region during the 20th century. Int. J. Climatol., 22, 1049-1074 Schmidli, J. and C. Frei, 2005: Trends of heavy precipitation and wet and dry spells in Switzerland during the 20th century. Int. J. Climatol., in press Seneviratne, S. I., P. Viterbo, D. Lüthi and C. Schär, 2004: Inferring changes in terrestrial water storage using ERA-40 reanalysis data: The Mississippi River basin. J. Climate, 17, 2039-2057 Van den Hurk, B., M. Hirschi, C. Schär, G. Lenderink, E. van Meijgaard, A. van Ulden, B. Röckel, S. Hagemann, P. Graham, E. Kjellström and R. Jones, 2005: Soil control on runoff response to climate change in regional climate model simulations. J. Climate, in press Vidale P.L., D. Lüthi, R. Wegmann, C. Schär, 2005: Variability of European climate in a heterogeneous multimodel ensemble. Clim. Change, submitted 24
Page 1: ABSTRACTS OF ORAL AND POSTER CONTRI
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Die mögliche Verschärfung des Hoc
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Floods in Europe - Loss experience
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Figure 2: Proportions of numbers an
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Peter Schmoker Berner Fachhochschul
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Some comments on the estimation of
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Discharge [m³/s] 4000 6000 8000 10
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Risk management of extreme flood ev
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ABSTRACTS 'Extreme Discharges' - CHR-KHR

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